In the summer of 1997, the US National Oceanic and Atmospheric Administration picked up a sound from deep beneath the Pacific. The sound seemed to come from an animal far larger than any we've ever seen. This was the Bloop.

The Bloop is one of about a half-dozen unexplained sounds that the NOAA's Acoustic Monitoring Project has picked up in its more than twenty years listening to the noises of the Pacific. While some of these sounds seem to have relatively obvious explanations, a few really are baffling, and they represent one of science's great unanswered mysteries. Let's now take a closer listen to the Bloop and five other strange underwater sounds.

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Back in the Cold War, the US Navy set up a series of massive arrays of microphones throughout the world's oceans. these, unsurprisingly enough, meant as a way to listen in on Soviet submarines, and they took advantage of a phenomenon known as the deep sound channel, an ocean layer where the speed of sound becomes virtually nothing and low-frequency soundwaves that enter the channel can become trapped, bouncing around in this layer for thousands of miles.

This phenomenon allowed the arrays of the Sound Surveillance System, or SOSUS, to be able to detect even relatively weak sounds from hundreds of miles away. With the end of the Cold War around 1990, the arrays' original 30-year mission came to an end and was replaced with a new civilian function of just generally monitoring the sounds the ocean. For the last twenty years, the NOAA and its Equatorial Pacific Ocean autonomous hydrophone array have been doing just that.

For the most part, it's not hard to identify the sounds that are emitted. Whales are a frequent source of low frequency noises, as are volcanic activity and iceberg movement, plus all the human-made devices still at work under the sea. These all have their own distinctive soundprint, so that there's rarely any question of where a sound came from. But every so often, the Acoustic Monitoring Project picks up a sound that defies explanation. Here are the six sounds that the NOAA officially considers unexplained. All of these have been sped up between 16 and 20 times their real speed so that we actually hear them.

The most famous of these sounds is this one, known as the Bloop. It was recorded in 1997 originating from a point about 1,500 miles west of the southern Chilean coast. It was powerful enough to be picked up on sensors located up to 3,000 miles away, making it one of the most powerful noises ever recorded underwater. The sound lasted for just over a minute and has not been detected since. It should be pointed out now that the NOAA has checked with the Navy and other groups to rule out human-made sources in this and the rest of these cases. We'll come back to other possible explanations for the Bloop in a little bit but let's first examine the other sounds.

This is Upsweep, which was first recorded in August 1991. Unlike most of the other sounds on this list, it can still be heard. While the noise is strongest in the spring and fall, it appears to be getting generally weaker over time. It's located somewhere deep in the South Pacific near Antarctica, located about 2,500 miles due west of the very southern tip of South America. It was initially thought that this sound might be created by fin whales, but in 1996 researchers Emile Okal and Jacques Talandier argued that there wasn't enough variation in the tone for it to be biological - whales wouldn't be able to communicate much if they only used these same tones over and over. They argued that this was some unusual acoustic phenomenon linked to volcanic activity in the region, perhaps the result of seawater and volcanic gas interacting and creating a resonance pattern. Sure enough, a French research vessel found volcanic seamounts in the region, which makes this the most likely explanation.

Let's move on to Slow Down, which was first recorded on May 19, 1997. Like Upsweep, the sound can still be heard several times each year. The sound was detected about 2,000 miles west of Peru, but its actual origin is much more southerly, and it's possible that the sound actually originates in the Antarctic. Its basic sound profile matches the sound of objects rubbing together in a massive friction event, such as icebergs calving or a sudden glacial movement. These seem like the most likely explanations for Slow Down, but as yet we haven't been able to identify any specific sources for these noises, so the mystery remains.

Next up is Train, so named because it recalls the sound of a distant train. This one was recorded on March 5, 1997, although we don't know exactly where the sound came from. The most likely explanation for this one, according to Christopher Fox, is the movement of ocean currents, as he explained in 2002: "Moving fluids generate vibrations, just like blowing air through a clarinet. If you have moving ocean water and the right conditions coming around a seamount or something, that could generate sound."

We know very, very little about Julia, which was recorded on March 1, 1999. The sound lasted all of fifteen seconds, and in that time it was picked up by every sensor on the Equatorial Pacific Ocean autonomous hydrophone array. Its origin is somewhere around 1,500 miles west of Peru's southern coast, but beyond that? We've got no idea what caused it.

Finally, we have Whistle, which was recorded on July 7, 1997. This one was only picked up by a single hydrophone located about 1,700 miles west of Costa Rica, and the precise origin of the sound is unknown. There aren't currently any preferred explanations for this sound.

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So, where does this leave us? We've got a pretty decent handle on the origins of Upsweep, Slow Down, and Train, although none of these can be considered confirmed explanations. Julia and Whistle are harder to pin down with a specific explanation, but they don't baffle scientists or inflame the imagination in quite the same way that Bloop has.

It's worth noting as a general principle that there's a big difference between things that are full-on unexplainable and others that are simply unexplained. While the former might force us to consider some pretty out there hypotheses in an attempt to make sense of what's going on, the latter is a more mundane kind of mysterious. We don't know what caused these sounds, but that's more a product of having precious little data to work with and the Pacific Ocean being a very, very big place. Indeed, the depths of the Pacific constitute the largest unexplored frontier on the planet. It would be amazing if we could explain everything we encounter in it.

But if there's one noise that is dangerously close to tipping over from unexplained to unexplainable, it's the Bloop. While ice calving has been thrown around as a possible explanation - its southerly location does make that a decently likely possibility - the profile of the sound far more closely matches that of an animal. And that's where the whole thing gets really strange.

If the Bloop was made by an animal, then it seemingly must be larger than any other known organism. Even the blue whale, whose record length is about 110 feet, would not be nearly big enough to account for the Bloop. Could such a leviathan exist? It's possible, and the Bloop might be considered the strongest evidence for such a beast...but it's also pretty much the only such evidence. There's not a shred of evidence to support the existence of what we might call a supergiant whale, and even with the entire Pacific Ocean to hide in, it's difficult to credit that a species that must continually come to the surface to breath could completely hide its existence.

The other possibility is some sort of massive squid. These creatures do serve as a catch-all for all that's still mysterious about the ocean depths, and our extremely limited firsthand knowledge of them makes it easier to believe a gigantic Blooping squid could maybe be hiding deep in the Pacific. There's a couple problems with this though.

First, the largest known squid is only about a third the length of the largest whale - there's an account of a 60-foot squid washing ashore in Newfoundland in 1878, but these claims were almost certainly the result of some spirited exaggeration. Plus, the current consensus is that squids don't have the organs necessary to create loud noises like whales do, so they may have to be eliminated entirely.

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That doesn't leave us with much room to maneuver. It may be worth splitting a hair and pointing out the animal in question wouldn't necessarily have to be larger than any other - just far, far better at making low frequency sounds. That doesn't get us any further to identifying the Blooper, but at least it relieves a bit of the pressure in having to find a 200-foot whale or something like it. Still, all we have to work on is a single, poorly understood noise from 1997, so unless we hear a new Bloop, all we're likely to have is speculation and guesswork.

Of course, I'd be remiss if I didn't mention the preferred fanciful explanation for the Bloop. It just so happens that the origin of the Bloop noise is located in the same general part of the southern Pacific Ocean as the location H.P. Lovecraft gave in his 1928 short story The Call of Cthulhu for the underwater, extra-dimensional city of R'lyeh. The coincidence isn't that spooky - the two locations are about a thousand miles apart - but the two are close enough together that Lovecraft fans have suggested the Bloop might well be the sound of a snoozing Cthulhu.

Honestly, at this point, it seems roughly as believable as any other explanation. It's really just nice to know the oceans are still keeping at least a few seriously hardcore mysteries.